Beating the old, dead horse....GMT800 disk rear axle swap

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1Matthias

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So to make a long story short, the rear axle on the FrankenTruck is a little less than pleased at the moment: seems the previous owners ignored a substantial pinion seal leak, ran it low, and now it's on the way out. I *could* rebuild it...but there's a pick and pull 6.0 GMT800 2500HD near me that has a G80 14 bolt FF in the ratio I need, with disk rear brakes that seems a tempting upgrade. Now, to summarize what I've learned and my questions:

Minor concerns/findings:
1 The spring perches will need to be cut off and rewelded moved slightly over, ditto(?) for the shock mounts. This doesn't scare me, no stranger to welding and fabrication.
2 The track width will be slightly different. Again, not particularly worried: I do have room under the utility bed for the added width, and it might help fill out the arches a bit better and make tire work feel less like spelunking. :)
3 Parking brake cables will need to be fabbed/mashed together. Again, not worried about doing this, seems an easy enough task.
4 The yokes are different on the GMT400 and GMT800 FF14 bolts - again, not an issue as I can either swap the ujoint in the truck, or swap the yoke on the axle.

Now, the big concerns...
1 Brake proportioning. I wish to retain ABS on this truck for a variety of reasons, so a simple delete is out of the question. I have the 1995+ style ABS unit with the lines coming in the rear (facing the firewall) style ABS block. Supposedly you can remove the proportioning/residual valve from the combination block as referenced here https://www.gmt400.com/threads/what-proportion-valve-to-use.52803/post-1160468 to convert it for disk/disk use. That post also referenced a disk/disk combination valve: does anyone know of a part number for this disk/disk combination valve?
2 Master cylinder selection. This truck has hydroboost as it was a 3500 originally/mechanically. Would I simply need to swap to a NBS/GMT400 disk/disk hydroboost master cylinder, or is there more nuance to it than that? I do not have quick uptake front calipers thanks to being a 1 ton, so that is not a concern.

I will be towing rather heavy loads regularly with this truck and live in a somewhat hilly location...so the added stopping power of disks would be greatly appreciated.

I greatly appreciate any and all help in advance, and if there's anything I've neglected to consider please let me know!
 

Schurkey

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there's a pick and pull 6.0 GMT800 2500HD near me that has a G80 14 bolt FF in the ratio I need, with disk rear brakes that seems a tempting upgrade.

1 Brake proportioning. I wish to retain ABS on this truck for a variety of reasons, so a simple delete is out of the question. I have the 1995+ style ABS unit with the lines coming in the rear (facing the firewall) style ABS block. Supposedly you can remove the proportioning/residual valve from the combination block as referenced here https://www.gmt400.com/threads/what-proportion-valve-to-use.52803/post-1160468 to convert it for disk/disk use. That post also referenced a disk/disk combination valve: does anyone know of a part number for this disk/disk combination valve?
Take it from the donor truck. Maybe take the entire ABS unit.

2 Master cylinder selection. This truck has hydroboost as it was a 3500 originally/mechanically. Would I simply need to swap to a NBS/GMT400 disk/disk hydroboost master cylinder, or is there more nuance to it than that?
Do you have hydroboost already? If not, take the entire hydroboost, or at least the master cylinder from the donor truck. Measure the piston size of the master even if you don't buy it with the axle. Get a master to fit your booster that has the same-diameter pistons as the donor truck.

Or adapt the hydroboost and master from the donor into your vehicle. I guess this is not a straight swap, some fab is needed (but you're good with that.)

There's a thread about adapting the GMT800 front discs to the GMT400 platform. Maybe upgrade all four corners?
 

BeXtreme

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So to make a long story short, the rear axle on the FrankenTruck is a little less than pleased at the moment: seems the previous owners ignored a substantial pinion seal leak, ran it low, and now it's on the way out. I *could* rebuild it...but there's a pick and pull 6.0 GMT800 2500HD near me that has a G80 14 bolt FF in the ratio I need, with disk rear brakes that seems a tempting upgrade. Now, to summarize what I've learned and my questions:

Minor concerns/findings:
1 The spring perches will need to be cut off and rewelded moved slightly over, ditto(?) for the shock mounts. This doesn't scare me, no stranger to welding and fabrication.
2 The track width will be slightly different. Again, not particularly worried: I do have room under the utility bed for the added width, and it might help fill out the arches a bit better and make tire work feel less like spelunking. :)
3 Parking brake cables will need to be fabbed/mashed together. Again, not worried about doing this, seems an easy enough task.
4 The yokes are different on the GMT400 and GMT800 FF14 bolts - again, not an issue as I can either swap the ujoint in the truck, or swap the yoke on the axle.

Now, the big concerns...
1 Brake proportioning. I wish to retain ABS on this truck for a variety of reasons, so a simple delete is out of the question. I have the 1995+ style ABS unit with the lines coming in the rear (facing the firewall) style ABS block. Supposedly you can remove the proportioning/residual valve from the combination block as referenced here https://www.gmt400.com/threads/what-proportion-valve-to-use.52803/post-1160468 to convert it for disk/disk use. That post also referenced a disk/disk combination valve: does anyone know of a part number for this disk/disk combination valve?
2 Master cylinder selection. This truck has hydroboost as it was a 3500 originally/mechanically. Would I simply need to swap to a NBS/GMT400 disk/disk hydroboost master cylinder, or is there more nuance to it than that? I do not have quick uptake front calipers thanks to being a 1 ton, so that is not a concern.

I will be towing rather heavy loads regularly with this truck and live in a somewhat hilly location...so the added stopping power of disks would be greatly appreciated.

I greatly appreciate any and all help in advance, and if there's anything I've neglected to consider please let me know!
Do you already have a 14BFF? The disk brakes don't give you more stopping power than the big drums, they dissipate heat better for spirited and heavy braking(like racing at a track) and they are less susceptible to fading out due to heavy water or mud immersion. If it was me, and I already had a 14BFF in it, I would get a rebuild kit and a yukon duragrip carrier to go in it. The absolute last thing I would want is a 14BFF with a G80 govbomb in it. I'd take an open carrier one over that and just put a grizzly locker in it.

My 1980 K25 originally came with an open diff 14BFF with the 13" drums. I just recently rebuilt everything including the wheel bearings and brakes on both ends while putting in a Grizzly locker. Runs great and has no issues towing my 6k lb trailer anywhere I want to go.
 

1Matthias

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Take it from the donor truck. Maybe take the entire ABS unit.


Do you have hydroboost already? If not, take the entire hydroboost, or at least the master cylinder from the donor truck. Measure the piston size of the master even if you don't buy it with the axle. Get a master to fit your booster that has the same-diameter pistons as the donor truck.

Or adapt the hydroboost and master from the donor into your vehicle. I guess this is not a straight swap, some fab is needed (but you're good with that.)

There's a thread about adapting the GMT800 front discs to the GMT400 platform. Maybe upgrade all four corners?
I can see about taking the ABS unit: when I was looking online, they seemed to be a *very* different design than the one on my truck. My ABS block has the combo valve bolted to the front and a separate part of the assembly, the newer styles seem to integrate it completely into the internals of the block. Now, if the new ABS block can be made electrically compatible with my older truck, I'm more than happy to bend and fabricate lines for the truck again: already did it once. https://i.ebayimg.com/images/g/NcwAAOSwY11kOLhB/s-l1600.jpg This appears to be the module on the JY example...and this is the one on my truck:
You must be registered for see images attach

The truck does indeed have hydroboost - I was planning on taking/measuring the master cylinder from the donor truck anyway - even though I've read the NBS masters have a larger mounting bore where they go into the booster, I can always chuck it in my neighbor's lathe and turn down the OD a hair, or chuck the front of the booster and open it up a bit. I do already have a slightly larger than normal bore master: this truck came with JD8 brakes from factory. I was considering upgrading the fronts, but *most* info I could find related to 4WD trucks, and mine is RWD only - and frankly, the front stopping power seems adequate even with the weight of this vehicle.
 

1Matthias

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Do you already have a 14BFF? The disk brakes don't give you more stopping power than the big drums, they dissipate heat better for spirited and heavy braking(like racing at a track) and they are less susceptible to fading out due to heavy water or mud immersion. If it was me, and I already had a 14BFF in it, I would get a rebuild kit and a yukon duragrip carrier to go in it. The absolute last thing I would want is a 14BFF with a G80 govbomb in it. I'd take an open carrier one over that and just put a grizzly locker in it.

My 1980 K25 originally came with an open diff 14BFF with the 13" drums. I just recently rebuilt everything including the wheel bearings and brakes on both ends while putting in a Grizzly locker. Runs great and has no issues towing my 6k lb trailer anywhere I want to go.
Yep, this truck came with an open diff 14BFF. As this truck is RWD only, it's not intended as an offroad etc rig: more just would appreciate a little extra traction on the occasional dirt/gravel road, or pulling out from jobsites. For my use, a late stock G80 seems the best value for money as although attractive I doubt I'd ever be getting full use out of an aftermarket locking solution. I understand about the stopping power: heat dissipation is an issue with some of the grades around here, not uncommon to see brake fires on one local mountain. It's also a personal preference, I am....not the biggest fan of drum brakes to be *brutally* honest. If this swap proves to be more of a headache than it's worth, I'll just rebuild my current axle - this seemed an decent way to kill 3 birds with one stone however.
 

Erik the Awful

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I will be towing rather heavy loads regularly with this truck and live in a somewhat hilly location...so the added stopping power of disks would be greatly appreciated.
You absolutely have to do this right. There's some real danger in mixing and matching brake parts.

there's a pick and pull 6.0 GMT800 2500HD near me that has a G80 14 bolt FF in the ratio I need, with disk rear brakes that seems a tempting upgrade.
Disc brakes aren't necessarily an upgrade. The benefit is that they don't need adjustments and servicing discs is much easier than drums. The problem with swapping in a disc brake rear is that disc brakes require a higher clamping pressure to achieve the same braking force as drums. Drums have a natural self-energizing effect that makes the pedal effort easier. Throwing a disc brake rear axle into a truck that had drums will lower the braking force you get from the rear, but since they also require more pressure, it takes away from the braking force you can achieve with the front discs. If you swap in the rear brakes, I'd also swap in the front brakes and master cylinder.

Master cylinder selection. This truck has hydroboost as it was a 3500 originally/mechanically. Would I simply need to swap to a NBS/GMT400 disk/disk hydroboost master cylinder, or is there more nuance to it than that?
This is the $50,000 question. Will the GMT800 master cylinder bolt to the GMT400 hydroboost? Or can you swap the GMT800 hydroboost into your truck? I don't know, I only have experience with one 3500, and it has a vacuum booster and a rusted out master cylinder.
 

arrg

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This is the $50,000 question. Will the GMT800 master cylinder bolt to the GMT400 hydroboost? Or can you swap the GMT800 hydroboost into your truck? I don't know, I only have experience with one 3500, and it has a vacuum booster and a rusted out master cylinder.
The GMT800 hydroboost master cylinder is 1.46" vs 1.25" for the GMT400 hydroboost master. The GMT800 master won't fit into the bore of the GMT400 hydroboost.
 

OutlawDrifter

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The absolute last thing I would want is a 14BFF with a G80 govbomb in it. I'd take an open carrier one over that and just put a grizzly locker in it.

Too my knowledge, the vast majority of horror stories are on the 10-bolt axles, not the 14B, when it comes to the G80 "Gov Bomb". And everyone of them that I've personally been around(it has never happened to one of my personal vehicles) has "exploded" from abuse and neglect.
 

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The GMT800 hydroboost master cylinder is 1.46" vs 1.25" for the GMT400 hydroboost master. The GMT800 master won't fit into the bore of the GMT400 hydroboost.
I saw something a while back where someone modded a GMT800 HB to install in a GMT400, I don't have a link anymore since I lost my last computer and lost the links to tons of stuff.
For a disc swap I think the 3500HD reservoir works perfect and it's a bolt on application, not sure if someone else mentioned it above.
Too my knowledge, the vast majority of horror stories are on the 10-bolt axles, not the 14B, when it comes to the G80 "Gov Bomb". And everyone of them that I've personally been around(it has never happened to one of my personal vehicles) has "exploded" from abuse and neglect.
14-bolt G80s are relatively stout. Came across a guy a little while ago who did grudge and sled pulling in a 320K mile K2500 and it still had the factory rear, all he had done was a Lugnut4x4 disc kit and a bigger diff cover. All stock on a 35" Super Swamper.
I only know of one guy who managed to kill about 4 or 5 of them in a period of time.

Funny enough that truck was a 6.5/NV4500, and the engine outlasted everything else. Ran for about 20 minutes at WOT with low oil before it finally died, and even after that it kept trying to crank.
 

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davkenrem

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I did a GMT800 Hydroboost swap on my GMT400. Got the Hydroboost and master cylinder from a 2002 1500 Z71 Suburban. The push rod on the the GMT800 is a different length than the GMT 400, I had to do a custom length one for my truck.

From an earlier post:
"My vehicle is a 97 K1500. The Pedal came from a 96 C3500. The Hydroboost came from a 2005 z71 Suburban (GMT800).


I installed a GMT400 ring on the GMT800 booster.

The stud on the GMT400 Hydroboost pedal has a stud with a shoulder at the base and is larger than the stud on the GMT400 Vacuum booster. I had to drill out the ring I welded on.

Welding on the ring was necessary because I was using the GMT800 booster and the rod length is too short on the for the GMT400.

I'm sure if your if your using a GMT400 pedal and booster it should all work. The vacuum pedal will not work with either the GMT400 or GMT 800 hydroboost. Note the location of the stud in my photo above."
 
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